Astronauts from the US space shuttle Atlantis prepared for an ambitious spacewalk to overhaul the Hubble space telescope and extend its working life.
The spacewalks come on the heels of an operation Wednesday during which astronauts plucked Hubble from orbit, maneuvering it into the cargo bay of Atlantis.
John Grunsfeld, 50, will lead the first of five spacewalks Thursday at 1216 GMT. Joining him will be Drew Feustel, a 43-year-old geologist on his first space mission.
Officials believe the overhaul will extend operations at least five years, long enough to finish the development and launch a more capable successor, the James Webb Space Telescope.
During a six- to seven-hour spacewalk, the two men will replace the Wide Field Planetary Camera-2, a 16-year-old workhorse imager, with the updated Wide Field Camera-3.
The new camera was designed to look deeper into the universe with observations in the ultraviolet and infrared regions of the electromagnetic spectrum.
As their final task, Grunsfeld and Feustel will replace the telescope's failing science computer.
The Science Instrument Command and Data Handling system experienced a partial electronic failure in late September. The breakdown prompted NASA to postpone plans to launch the Hubble mission in October so engineers could prepare a replacement.
When the overhaul is complete, Hubble should have new batteries and gyroscopes, rejuvenating the electrical and pointing systems.
On Wednesday, astronaut Megan McArthur grappled the 13.2-meter (43-foot) telescope with the shuttle's robot arm at 1714 GMT, after Atlantis commander Scott Altman maneuvered his spacecraft within 10 meters (35 feet) of the scientific icon.
"Houston, Atlantis, Hubble has arrived on board," Altman radioed Mission Control.
The two spacecraft sailed 560 kilometers (350 miles) above Australia at the time of the capture. After the grapple, McArthur carefully hoisted the observatory toward a rotating work platform in the rear of the shuttle's cargo bay.
The big telescope will remain anchored to the platform for the next six days.
Late Wednesday, a small fragment from a satellite destroyed in 2007 Chinese anti-satellite test passed an estimated three kilometers (1.8 miles) from Atlantis without incident.
Astronauts aboard the shuttle were told to prepare for evasive action, but it proved not to be necessary, NASA officials said.
Meanwhile, mission managers concluded the shuttle's heat-shielding weathered Monday's launch without significant damage, freeing astronauts to focus on refurbishing Hubble, said LeRoy Cain, the head of the management team.
Small, shallow gouges were found across four heat shielding tiles but NASA managers decided they posed not threat to the mission.
Wednesday's rendezvous operation grew more challenging when a communications problem kept the Atlantis crew from seeing the results of positioning commands they transmitted to the telescope.
Hubble's Maryland command center monitored the commands instead, relaying the results to the astronauts.
Altman flew the final kilometer of the encounter manually, gingerly easing Atlantis closer to the telescope from below with the assistance of shuttle pilot Greg Johnson and Mike Good.
The astronauts later scanned the Hubble's exterior with cameras on the robot arm, finding it in good shape despite signs of weathering from ultraviolet radiation and impacts from space debris.
"It's an unbelievably beautiful sight," gushed Grunsfeld, an astronomer making his third trip to the space telescope. "Amazingly, the exterior of Hubble, an old man of 19 years in space, still looks in terrific shape."
Shuttle astronauts had not seen nor worked on Hubble since March 2002.
Hubble, a cooperative project between NASA and the European Space Agency, has been refurbished four times since its launch in 1990.
Source: AFP American Edition